1. Field of the Invention
The present invention relates to a printed wiring board on which an electronic component provided with a heat sink is mounted, a printed circuit board provided with the electronic component, and a method for manufacturing a printed circuit board.
2. Description of the Related Art
The electronic component which is mounted on the printed wiring board and has a lead electrode has a heat sink provided on its opposite face to the printed wiring board. In addition, the printed wiring board has a heat transfer pattern formed on its opposite face to the electronic component. The heat sink of the electronic component and the heat transfer pattern of the printed wiring board are connected by a solder paste or the like.
On the other hand, in the case of an electronic component which is provided with a heat sink and is manufactured according to a standard of a conventional lead frame (semiconductor package of QFP type, SOP type and the like), a gap (stand-off) is formed between a bottom face of the lead electrode and a bottom face of the heat sink of the electronic component. Generally, the stand-off is set at 100±50 μm according to a package standard of JEDEC and the like.
This electronic component provided with the heat sink is often used for a component with a narrow pitch, which has a pitch of 0.4 mm or the like between leads. Accordingly, in order to prevent a bridge between the leads, it is necessary to decrease the thickness of the solder paste as thin as possible which connects the heat sink of the electronic component to the heat transfer pattern of the printed wiring board. As a result, the value of the stand-off of the electronic component varies because of a tolerance in manufacture. When the value of the stand-off becomes equal to or thicker than the thickness of the solder paste in particular, non-connection has occasionally occurred between the heat sink of the electronic component and the heat transfer pattern of the printed wiring board. In addition, when the solder paste is printed, the solder paste results in being scraped by a squeegee. Accordingly, dispersion occurs also in a print volume of the solder paste, and non-connection has occasionally occurred between the heat sink of the electronic component and the heat transfer pattern of the printed wiring board.
Then, a technology of dividing a heat transfer pattern with a solder resist is described in Japanese Patent Application Laid-Open No. 2006-303392. The technology in Japanese Patent Application Laid-Open No. 2006-303392 includes applying the solder paste onto the whole face of the heat transfer pattern, using such a cohesive effect that the solder paste in a portion on the solder resist moves to an exposed portion of the heat transfer pattern and rises due to the surface tension when the solder paste is fused, and thereby suppressing the poor connection.
In addition, Japanese Patent No. 3639505 describes a technology of radiating heat to a conductor which is arranged on an inner layer or a surface layer at an opposite side of a surface layer on which the electronic component is mounted, through a through hole. In order to prevent fused solder from flowing into the through hole, the technology in Japanese Patent No. 3639505 tries to provide the through hole in a portion of a solder resist, and to print the solder paste only on a solder area in which a heat transfer pattern is exposed while avoiding the portion of the solder resist.
However, the technology in Japanese Patent Application Laid-Open No. 2006-303392 does not have a through hole for releasing the heat of the electronic component to the printed wiring board, and accordingly does not show a sufficient heat dissipation effect.
In addition, in Japanese Patent No. 3639505, the solder paste is printed only on the solder area, and accordingly there is not such a cohesive effect that the solder rises due to the surface tension when having been fused. In other words, it is likely that non-connection occurs between the heat sink of the electronic component and the heat transfer pattern of the printed wiring board, in an electronic component having a large stand-off because of the dispersion in values of the stand-off.
Accordingly, the present invention provides a printed wiring board which can enhance the heat dissipation of the electronic component while enhancing the connectability of the heat transfer pattern with the heat sink of the electronic component, when the electronic component having the heat sink is mounted thereon; a printed circuit board; and a method for manufacturing a printed circuit board.